1. Field of the Invention
The present invention relates generally to the structure of radially expansible lumenal prostheses, including stents and grafts. More particularly, the present invention relates to the provision of articulation structures for the construction of flexible and pseudo-flexible prostheses and the provision of end structures for the construction of atraumatically deliverable prostheses.
Lumenal prostheses are provided for a variety of medical purposes. For example, lumenal stents can be placed in various body lumens, such as blood vessels, the ureter, the urethra, biliary tract, and gastrointestinal tract, for maintaining patency. Lumenal stents are particularly useful for placement in pre-dilated atherosclerotic sites in blood vessels. Lumenal grafts can be placed in blood vessels to provide support in diseased regions, such as aortic abdominal, and other aneurysms.
Both stent and graft prostheses must meet certain mechanical criteria to function successfully. In particular, such prostheses should be at least partly flexible or articulated (such as rigid sections that articulate relative to one another) over their lengths so that they may be advanced through tortuous body lumens, such as those of the coronary vasculature. In addition, the prostheses should preferably maintain their original length or foreshorten minimally when the prostheses assume an expanded configuration. Further such prostheses must have sufficient mechanical strength, particularly hoop strength, in order to mechanically augment the lumenal wall strength and thus assure lumen patency. The ability to meet these requirements is severely limited in the case of cylindrical endolumenal prostheses which are delivered in a radially constrained or collapsed configuration. Such prostheses must radially expand at a target site within the body lumen, so any adaptations which are intended to enhance flexibility will not interfere with the ability to radially expand or to maintain strength once expanded.
Prior lumenal prostheses often have structures which present a risk of injury as they are endolumenally delivered (i.e. tracked) to and/or released at a target site within a patient's body lumen. In particular, many vascular stents comprise a plurality of circumferentially connected and spaced-apart longitudinal elements which deform circumferentially as the stent is radially expanded. The Palmaz stent described in U.S. Pat. Nos. 5,102,417 and 4,776,337, is typical of such stents. When these prostheses are flexed or articulated during delivery or tracking, these longitudinal elements of conventional prosthesis tend to create a phenomenon known as “fishscaling” which occurs when these elements distributed along the length of the prosthesis protrude outward from the surface of the prosthesis. Such stents exhibit poor “tracking” characteristics, where “tracking” is defined as the ability to pass smoothly through tortuous pathways. These protruding elements along the length of the prosthesis increase the likelihood that the prosthesis will dig into or otherwise engage the wall of the body lumen during delivery and even arrest the progress of the prosthesis and its delivery system to the diseased region (or target site). Additionally, a lesser-known phenomenon called “flaring” occurs when the longitudinal elements of the distal or proximal end of the prosthesis are bent outward to assume a crown-like configuration due to bending forces placed on these elements as the prosthesis passes through tortuous body passageways. Flaring can create the same deleterious effects as the previously described fishscaling phenomenon, injuring or traumatizing the blood vessel wall as the prosthesis is delivered or tracked within the blood vessel.
A separate problem in stent construction and deployment relates to the ability to detect the stent fluoroscopically during the deployment procedure. Stainless steel, the most common stent material, is generally radiolucent, i.e. it is minimally visible under x-rays and permits fluoroscopic examination therethrough. Advantageously, such stents do not interfere with subsequent fluoroscopic examination of the treated region of the body lumen, such as six-month-followup examinations. They are, however, much more difficult to accurately position within the lumen due to their radiolucency. To increase radiopacity, these prostheses may be manufactured from radiopaque materials such as tantalum, platinum, or nickel titanium (NiTi). Alternatively, the entire prosthesis may be plated or coated with a uniform layer of radiopaque material to improve prosthesis visualization as disclosed in commonly assigned, co-pending U.S. patent application Ser. No. 08/691,661, filed Aug. 2, 1996, the complete disclosure of which is incorporated herein by reference. Although these methods address the issue of radiopacity, uniform layers of such materials or prostheses made entirely of such materials typically improve radiopacity at the cost of reduced visibility of tissue inside the prosthesis.
For these reasons, it would be desirable to provide improved stents and other lumenal prostheses. In particular, it would be desirable to provide improved lumenal prostheses and methods for their endolumenal placement, where the prostheses can be delivered or tracked to a target site within a body lumen without traumatically engaging the walls of the body lumen. Preferably, the prostheses will have elements which minimize “fishscaling” with its concomitant risk of injury or even retention within the body lumen. Such prostheses may also be provided with minimally traumatic end rings in order to reduce the risk of lumenal injury as the prostheses is both delivered and radially expanded within the body lumen. Optionally, the ends of the prostheses may incorporate stiffening elements which reduce the occurrence of prosthesis flaring or trumpeting during tracking through tortuous body lumens. Additionally, the prostheses will be radially expansible at the target location, and will preferably retain both their cylindrical configuration and flexibility or ability to articulate after expansion. Such prostheses should further have sufficient hoop strength and other mechanical characteristics so that they may effectively function as stents in maintaining lumenal patency and/or grafts in enhancing lumenal wall strength. To more precisely direct scaffolding force at a diseased site, the prosthesis may have specific expanded configurations. Furthermore, it would be desirable to provide improved prostheses having radiopacity characteristics which permit visualization of the prosthesis both during tracking and deployment as well as visualization of tissue within the lumen during subsequent angiographic followup after deployment. The present invention will provide at least some of the desired improvements.
2. Description of the Background Art
Vascular stents comprising multiple segments joined by axial hinge structures are described in U.S. Pat. Nos. 5,195,984; 5,104,404; and 5,102,417 and European Patent Publication EP 540 290. Other stent structures are described in U.S. Pat. No. 5,282,824, European Patent Publication EP 481 365; and Canadian Patent Publication 2,079,944. U.S. Pat. No. 4,776,337 describes the Palmaz stent which consists of multiple longitudinal box elements joined to each other by short circumferentially oriented tabs and usually having at least two such sections joined longitudinally by a single short beam as shown in U.S. Pat. No. 5,195,984.